Our goals are to define in molecular terms the organization and regulated expression of rabbit immunoglobulin genes. Biologically active, physically intact mRNAs for rabbit heavy and light chains were prepared from rabbit spleens: cell free synthesis of allotype-specific gamma, and light chains was demonstrated by immunoprecipitation. Clones carrying cDNAs encoding gamma, kappa and mu sequences were identified. An unexpectedly high nucleic acid sequence homology between the 3' untranslated (UT), sequence of b4 and b5 cDNAS was observed. A cDNA copy of mu mRNA, including 5' UT, leader, VH, D, J, Cmu, secreted tail piece and 3' UT is almost completely sequenced. The VH sequence contains codons for amino acids characteristic of the VHa2 allotype. cDNA probes have identified lambda, kappa, gamma and mu mRNAs by Northern blot analyses. mRNAs that hybridize to kappa and lambda cDNA probes have also been detected in total RNAs prepared from cultured splenocytes. The high degree of homology of rabbit and mouse lambda light chain sequence was confirmed by cross-hybridization of mouse lambda1 cDNA to rabbit mRNA. Different mRNAs encoding one or more forms of mouse IgD have been identified and characterized. Our cloned and characterized cDNA probes allow us to study genomic DNA organization and regulated mRNA expression.